The present invention relates to a multicolor image forming apparatus and, more particularly, to an image transfer structure for transferring an image from a photoconductive element to an intermediate image transfer body.
In a multicolor image forming apparatus capable of forming an image in full color or two or more colors, images sequentially developed in different colors on a photoconductive element are transferred to an intermediate image transfer body one above another and then transferred to a paper sheet or similar recording medium at the same time. The intermediate image transfer body is often implemented as a belt passed over a plurality of pulleys and long enough to accommodate an image of maximum size available with the apparatus. The prerequisite with this kind of image forming apparatus is that the images of different colors be transferred to the belt in accurate register with one another. To meet this requirement, there has been proposed a structure which, on completing the transfer of an image of particular color, moves the belt in the same direction but at a speed higher than the speed necessary for image transfer and again moves the belt at the speed necessary for image transfer as it approaches an image transfer position, thereby causing the leading end of the image present on the belt to meet that of the image present on the photoconductive element. There has also been proposed a structure which, on completing the transfer of an image of a particular color, moves the belt in the opposite direction for thereby bringing the image on the belt into register with the image on the photoconductive element. In any of these structures, the belt is movable toward and away from the photoconductive element. Specifically, among a plurality of rollers over which the belt is passed, a roller facing the photoconductive element may be moved toward and away from the element, as disclosed in Japanese Patent Laid-Open Publication No. 164372/1981. Alternatively, a support member supporting all of the plurality of rollers may be implemented as a pivotable member whose side facing the photoconductive element is movable toward and away from the element, as taught in Japanese Utility Model Laid-Open Publication No. 109963/1988.
The conventional structures, however, have various problems left unsolved, as follows. When only one of the rollers is moved toward and away from the photoconductive element, the tension of the belt decreases on the movement of the belt away from the photoconductive element. This causes the belt and the roller to slip on each other when, for example, the belt is driven when spaced apart from the photoconductive element, dislocating the image transferred to the belt from the next image formed on the element. When the support member is bodily moved in a pivotal motion, a change in the load during the movement is apt to prevent the image transferred to the belt from being brought into register with the image on the photoconductive element.